The present invention is directed generally to carrier suppression in a proximity contactless card system.
The basic components of a proximity contactless card system are the contactless reader and a proximity card. The contactless reader is a single or dual loop reader coil connected to an electronic circuit.
The proximity card has an inductive coil and an integrated circuit connected to ends of this coil.
A proximity contactless system is based on the concept of magnetic coupling, which is the principle that current flowing in one circuit can induce current flow in another circuit through a radio frequency (RF) field generated in the space between the circuits.
In operation, the reader coil transmits to the proximity card a carrier signal, which generates the radio frequency (RF) field to supply the proximity card with power, and data, which is achieved by amplitude modulation of the carrier signal. The proximity card transmits data by modulating the carrier, and this modulated signal is detected by the reader coil.
More specifically, the combination reader-proximity card behaves as a transformer. An alternating current (i.e., carrier signal) passes through the reader coil in the reader to create the RF field, which induces a current in the proximity card coil. The proximity card converts the RF field generated by the reader coil into a DC voltage by means of a diode rectifier. This DC voltage is used to power the proximity card's internal circuits. The proximity card also modifies the received carrier signal in a way corresponding to data on the proximity card, and retransmits the modified carrier signal back to the reader. The modified carrier signal of the proximity card is coupled with the reader coil of the reader. A voltage divider, typically capacitive or resistive, and connected between the reader coil and the receiver input of the reader circuitry, dampens the high voltage of the modified carrier signal to a limited value.
Some proximity contactless systems have separate transmission and reception coils. In such systems, in order for the reader to be able to read the data transmitted on the modified carrier signal by the proximity card coil, the carrier signal field from the reader coil should cancel or suppress the carrier signal field from the proximity card coil. Field cancellation is simply achieved by inducing two voltages with the same amplitude but opposite direction (180° phase shift).
By way of example, carrier suppression is realized in a test apparatus for a contactless proximity card as defined in ISO/IEC (International Organization for Standardization/International Electrotechnical Commission) 10373-6. The test apparatus has a proximity coupling device (PCD) (transmitter coil) coaxially located between two parallel sense coils (reception coils). An RF amplifier has to be used in most cases to produce the required magnetic field strength of the PCD. The sense coils are connected to a balance circuit. An oscilloscope connected to a probe on the balancing circuit measures the modulated response signal strength of the proximity card.
Each of the sense coils is located at the same fixed distance of 37.5 mm from the PCD coil, so that induced voltages in the sense coils are canceled (180° phaseshift). Without the proximity card placed in a position at one of the sense coils, there is no carrier signal readable. During testing, the proximity card is placed at one of the sense coils, and the proximity card generates its own field due to current flow in the proximity card coil.